1. Field of the Invention
The invention pertains to the field of tensioners for imparting tension to a chain, belt or the like. More particularly, the invention pertains to a tensioner having a ratchet mechanism, including a slider, to prevent a plunger""s retraction when hydraulic pressure is decreased.
2. Description of Related Art
A hydraulic tensioner generally includes a housing, a hollow plunger that is fitted slidably into a bore formed in the housing, where the plunger is biased in a protruding direction by a spring, and a fluid chamber defined by the bore of the housing and the plunger. During operation, the force is exerted by a chain or belt on the distal end of the plunger, which balances the elastic force of the spring and hydraulic force in the fluid chamber.
When a force is exerted by a chain on the distal end of a plunger, an inadequate amount of hydraulic pressure is present in the chamber. This force is exerted at the time of engine start. As a result, the plunger is easily forced to retract into the housing, causing noise or oscillation. In order to prevent such a retraction of a plunger, various kinds of hydraulic tensioners with ratchet mechanisms have been proposed, such as shown in Japanese patent application laying-open publication Nos. 2000-136856 and 2001-304360.
The hydraulic tensioner shown in Japanese Publication No. 2000-136856 includes a rack supported translatably in a longitudinal hole formed in a housing, a ratchet fitted slidably in a lateral cavity formed in the housing, a rack which is engaged by the ratchet, and a spring fitted in the cavity, which biases the ratchet in an engaging direction with the rack. The ratchet moves in a the direction perpendicular to the movement of the rack at the time of protrusion of the plunger. One problem associated with this tensioner is that the rack is discrete from the plunger, such that the tensioner has an increased number of components. Additionally, in the case of the chain having a relatively long center distance, there exists a demand for increasing a backlash of a ratchet mechanism in some degree, but the above-mentioned tensioner has a drawback in that the backlash of the ratchet mechanism cannot be enlarged more than the backlash between the rack and the ratchet.
Japanese Publication No. 2001-304360 shows a hydraulic tensioner that includes a rack groove formed on the circumference of a piston. A claw member is adapted to engage the rack groove and slidably fits in a lateral hole. The lateral hole extends in a direction that crosses the axial centerline of the piston in the housing, via a predetermined clearance with an inside wall surface of the lateral hole. The tensioner also includes a spring that fits in the lateral hole that biases the claw member in the engagement direction with the rack. The claw member moves in the direction diagonal to the movement of the rack teeth at the time of protrusion of the piston. However, the amount of movement of the claw member does not fully contribute to the increase of the backlash. In this tensioner, it may be possible to enlarge the backlash of the ratchet mechanism more than the backlash present between the rack groove and the claw member by providing a predetermined clearance between the claw member and the inside wall surface of the lateral hole. However, increasing the backlash may cause chattering due to too great a clearance between the claw member and the lateral hole. Thus increasing the amount of clearance does not increase the backlash of the ratchet mechanism an adequate amount.
The present invention provides a hydraulic tensioner with a ratchet mechanism, that can reduce the number of components, ensure an adequate backlash, securely prevent a plunger""s retraction, and address the above problems found in the prior art.
A hydraulic tensioner according to one aspect of the present invention includes a housing having a central bore that opens at one end thereof, a hollow plunger slidably received in the bore of the housing, the plunger having an inside space that defines a fluid chamber with the bore and rack teeth that are formed at least at a portion of the outer circumference of the plunger. The rack teeth formed on the outer circumference of the plunger, eliminating the need to provide a separate rack member and plunger, thereby reducing the number of components of the tensioner.
The tensioner further includes a first biasing member that biases the plunger in a protruding direction, a wedge-shaped slider adapted to permit travel of the plunger in the protruding direction but to prevent travel of the plunger in a backward direction by developing a xe2x80x9cwedge-effectxe2x80x9d at the time of retraction of the plunger, and a second biasing member that biases the slider in a direction of engagement of the ratchet portion with the rack teeth. The slider has a ratchet portion engageable with the rack teeth of the plunger on a first side surface. The slider is slidable in a direction intersecting an axial centerline of the plunger along an inclined surface formed on a second side surface opposite the first side surface.
The housing has a supporting block formed with a slider housing portion, the slider housing portion has an inclined slide surface that the inclined surface of the slider slidably contacts, and a second biasing member biases the slider in such a way that the slider moves along the inclined slide surface. The slider housing portion has a stop surface adapted to contact a rear side surface of the slider to block travel of the slider in a backward direction. Thereby, the movement of the slider, and thus the plunger during retraction of the plunger can be securely prevented. The slider housing portion also has a front facing wall oppositely disposed to a front side surface of the slider, and one end of the second biasing member contacts the front facing wall and the other end thereof contacts the front side surface of the slider. At the time of movement of the slider, the slider is guided by the inclined slide surface formed in the slider housing portion of the supporting block under the action of the force of the second biasing member, thus making the movement of the slider smoothly.
The second biasing member may be a coil spring where the axial centerline is disposed parallel to the inclined side surface of the slider housing portion. In this case, the biasing force of the coil spring can be transmitted to the slider without any loss of contact between the inclined surface of the slider and the slide surface of the supporting block maintained. Thereby, the slider and the second biasing member are integrated and housed inside the supporting block, thus facilitating assembly of the tensioner.
When the plunger travels in the protruding direction during operation of the tensioner, the slider slides along the inclined surface through engagement between the rack teeth of the plunger and the ratchet portion of the slider, thereby increasing the whole backlash, correspondingly to the amount of sliding movement, allowing an adequate backlash to be secured. The plunger is prevented from retracting by a xe2x80x9cwedge-effect,xe2x80x9d which is present at the time of backward movement of the plunger by a wedge shaped slider.
The housing further contains a first through hole adapted to insert a disengaging pin thereinto between the rear side surface of the slider and the rear stop surface of the slider housing portion in order to disengage engagement of the ratchet portion of the slider with the rack teeth of the plunger. Also, the housing has a second through hole adapted to insert a retaining pin thereinto between the front side surface of the slider and the front facing wall of the slider housing portion in order to retain the plunger in a retracted state. Engagement of the slider with the rack teeth of the plunger can be easily disengaged by inserting the disengaging pin into the first through hole. By this disengagement, a worker can easily push the plunger into the housing before transportation of the tensioner. Then, from this condition, by inserting the retaining pin into the second through hole, the retracted condition of the plunger can be maintained with ease, thereby facilitating assembly of the tensioner into a chain. Additionally, after assembly into the chain, the tensioner is placed in an operatable state by removing the retaining pin.
The disengaging pin and the retaining pin are preferably the same pin with a tapped distal end. In this case, insertion of the pin into a clearance between the rear end surface of the slider and the stop surface of the slider housing portion, and into a clearance between the front end surface of the slider and the wall surface of the slider housing portion can be conducted with ease. Also, the use of only one pin as a disengaging pin and a retaining pin decreases the number of components.
A check valve may be provided at a bottom portion of the central bore of the housing to permit fluid flow into the chamber but to block reverse flow of fluid, thereby, securely preventing the retraction of the plunger.